1. Technical Field of the Invention
The invention relates generally to communication systems; and, more particularly, it relates to communication systems that may be affected by undesirable interference.
2. Description of Related Art
Signal processing within communication systems having a communication channel, in an effort to improve the quality of signals passing through the communication channel, has been under development for many years. In the past several years, emphasis has moved largely to the domain of digital communication systems that modulate bit streams into an analog signal for transmission over a communication channel. This channel can be a variety of channel types. Many different approaches are employed in the prior art to try to minimize or substantially reduce the effects of interference that may be introduced into a signal that is transmitted across a communication channel. In particular, the prior art approaches that seek to perform cancellation of interference that occupies a small number of signal dimensions in a signal are typically deficient for a number of reasons as is briefly referenced below. One particular type of interference that these prior art schemes seek to minimize is the narrowband interference that is sometimes referred to as ingress interference. Another type of interference occupying only a subset of the dimensions used by the signaling waveform is the interference of impulse/burst noise. Yet another type of interference that may be problematic is within the code division multiple access (CDMA) context when the interference is on a small number of codes. In the present context, the term “code,” “spreading code” or “despreading code” refers to a sequence of chips which are used to spread or despread a signal, such as in a spread-spectrum or CDMA system. This usage should not be confused with the language used to describe forward error correction (FEC) coding.
One of the main methods employed in the prior art to eliminate narrowband interference is the use of a notch filter. This solution is sufficient in some applications, but the notch filter itself oftentimes causes distortion of the desired signal. In the CDMA context, this distortion is called inter-code interference (ICI). Then, another means must oftentimes be included to remove the very ICI that has been introduced by the notch filter. One way to do this is to de-spread the signal. Then, hard decisions are made using the de-spread signal. The hard decisions are then respread and passed through the notch filter and subtracted from the original signal to remove the estimated distortion. In some instances, this process is repeated numerous times to try to achieve an adequate result. These prior art approaches described above are deficient in that they suffer the effect of error propagation. The decision circuit is prone to make incorrect decisions, requiring many iterations before the process converges, if it ever converges at all.
Similar problems exist for iterative methods to remove impulse or burst noise. The excision of the chips associated with the impulse or burst noise may be thought of as a time-domain filter. This filter itself causes distortion of the desired signal. In the CDMA context, this distortion is called inter-code interference (ICI). Then, another means must oftentimes be included to remove the very ICI that has been introduced by the filter. One way to do this is to de-spread the signal. Then, hard decisions are made using the de-spread signal. The hard decisions are then respread and passed through the filter and subtracted from the original signal to remove the estimated distortion. In some instances, this process is repeated numerous times to try to achieve an adequate result. These prior art approaches described above are deficient in that they suffer the effect of error propagation. The decision circuit is prone to make incorrect decisions, requiring many iterations before the process converges, if it ever converges at all.
Various techniques have been used to overcome problems caused by burst noise interference. These techniques include Forward Error Correction (FEC) coding, Automatic Retransmission reQuest (ARQ) operations, and time spreading. FEC coding includes adding coded (or parity) bits in the transmitter. At the receiver, the parity bits are used to reconstruct the missing pieces of the signal that resulted from degradation by burst noise. An example of FEC coding is block coding such as Reed-Solomon coding, in which an N-symbol block is sent consisting of k information symbols and N−k parity symbols. A burst of noise corrupting up to T=(N−k)/2 Reed-Solomon symbols can be corrected, and if erasure decoding is used, up to 2T Reed-Solomon symbols can be marked for erasure and the information is still recovered. With ARQ operations, error detection at the receiver determines missing data packets (or portions thereof) that were affected by the burst noise and causes retransmission of such missing data packets. ARQ operations introduce latency due to the requirements of lost data detection and retransmission.
Time spreading in Code Division Multiple Access (CDMA) systems, e.g., in an S-CDMA DOCSIS 2.0 system, spreads each data symbol across a time period. A given noise burst therefore may appear only during a partial duration of the data symbol. The effect of the noise burst can be repaired by coding and/or other techniques. Time spreading has a limited range over which it operates effectively, since burst noise that is very strong, that is of long duration compared to a spread symbol (e.g., many chips), or that corrupts more FEC symbols than the FEC is capable of correcting, can overwhelm the communication system.
Further limitations and disadvantages of conventional and traditional systems will become apparent through comparison of such systems with the invention as set forth in the remainder of the present application with reference to the drawings.